That is not true at all. You have Rails at your disposal but if you want to break free of that and use simple ruby classes and handle request/response on your own -- you are very much free to do that.

Perfect example is running Sinatra inside of Rails: https://jacobbednarz.com/posts/sinatra-in-rails

Again: ultimately you have a Rack (ruby, https://rack.github.io) webserver interface that has a Rails app mounted inside of it.

"Rails" is just some code that is running. You can choose to defeat certain parts of it, mount other stuff on the Rack server, turn rails off entirely and just use Activerecord, etc...

I do it all the time.